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Before I continue, please avoid buying an O'Reilly Nutshell book expecting it to teach you about the topic it is intended for. As far as I've worked with them, these books are not intended as a do-all be-all that other references want to be. The information introducing you to XML is sparse, so if you don't know anything about XML, get another book. I recommend XML: A Primer by Simon St. Laurent; it is an excellent learning tool, and though it doesn't go into all the detail the XML standard can go into (no book I've found can do such), it provides the user with understanding of XML.

XML in a Nutshell is what I use when I've forgotten how to use a certain aspect of XML detail. As a reference and a second book on XML, nothing comes close